(1) Field of the Invention
The present invention relates to a Raman optical amplifier, an optical transmission system using the Raman optical amplifier, and a Raman optical amplification method. Particularly, the present invention relates to a technique suitable for improving optical gain controllability in a specific wavelength or wavelength band.
(2) Description of Related Art
As optical amplifiers used in wavelength division multiplexing transmission systems, well known are an optical amplifier [erbium-doped optical fiber amplifier (EDFA) or the like] doped with a rare earth element, and a Raman optical amplifier exploiting stimulated Raman scattering effect.
WDM signals are heretofore intensively amplified by an EDFA in a repeater amplifier. However, amplification by the EDFA can yield high gain, but generates a relatively large amount of noise components such as ASE (Amplified Spontaneous Emission) light and the like, which is one of factors that limit the transmission distance in the whole system. Namely, even though the optical receiving terminal can receive an optical signal transmitted for a long distance at sufficient level (power), there occurs a phenomenon that the optical receiving terminal cannot normally demodulate the signal because the optical SNR (Signal to Noise Ratio) representing the quality of the received optical signal is poor.
In order to avoid such phenomenon, a Raman optical amplifier is placed before the EDFA, for example, to compensate a part of the transmission loss caused by the optical transmission line (optical fiber) by the Raman optical amplifier, the EDFA then intensively amplifies the optical signal. An advantage of such hybrid configuration, which is a combination of an EDFA and a Raman optical amplifier, is that it can amplify signals with less noise than the EDFA although yielding smaller gain than the EDFA because the Raman optical amplifier is an optical amplifier of a distributed amplification type distributively amplifying optical signals using the optical transmission line. Therefore, the hybrid configuration has an effect of improvement of the optical SNR as compared with a system configured with only the EDFA.
In the WDM transmission system, when the number of repeating amplifiers is increased, the optical SNR in a channel having a low gain is deteriorated, whereas the wavelength in a channel having a high gain is deteriorated due to nonlinear optical effect and the like. Accordingly, it is necessary to keep the optical amplifier gain in each channel constant.
The Raman optical amplifier inputs a plurality of pump lights at different wavelengths to an optical fiber (transmission line fiber), which is a nonlinear medium, to realize wide-band optical amplification that can be applied to a WDM transmission system. Namely, since it is known that the peak of the stimulated Raman scattering effect (Raman gain) generates at a wavelength shifted away from the pump light wavelength toward the longer wavelength by about 100 nm (nanometer) (at frequency of 13.2 THz), a plurality of pump lights at different wavelengths are used to widen the wavelength band that can be used as the main signal wavelength.
As other techniques relating to stimulated Raman scattering effect, there are techniques proposed in patent documents 1 and 2 below.
The technique in the patent document 1 relates to a wavelength division multiplexed optical transmitter. An object of the technique is, in a WDM transmission system using a plurality of wavelength bands, to decrease deterioration of the transmission characteristic due to non-degenerative four wave mixing occurring among signal lights in different wavelength bands, and cross phase modulation, decrease attenuation of signal lights at shorter wavelengths due to stimulated Raman scattering effect, and effectively use the bandwithout generating a dead band when a plurality of signal lights in different wavelength bands are coupled.
According to the technique described in the patent document 1, when a signal light in a short wavelength band (1530–1565 nm) is coupled with a signal light in a long wavelength band (1565–1605 nm) and sent to an optical fiber transmission line, the signal lights are polarization-combined so that the polarizations (directions of electric fields) of the signal lights in the respective wavelength bands are orthogonal. Since the signal light in a short wavelength band and the signal light in a long wavelength band are polarization-combined as above, the dead band at stake generating when a WDM filter is used does not generate, thus effective use of the band becomes possible. It becomes also possible to decrease deterioration of signals due to non-degenerative four wave mixing among the bands or cross phase modulation, and decrease the excessive loss of the signal light in the short wavelength band due to stimulated Raman scattering.
Since a polarization of two signal lights in one wavelength band and a polarization of one signal light on the other wavelength band are orthogonal among three signal lights relating to generation of non-degenerative four wave mixing, the generation efficiency of the non-degenerative four wave mixing becomes one-fourth. Since polarizations of two signal lights relating to cross phase modulation are orthogonal, the generation efficiency of the cross phase modulation becomes two-third. Since a polarization of a pump light and a polarization of a signal light to be amplified are orthogonal, the generation efficiency of stimulated Raman scattering becomes zero. The efficiency of attenuation of signal lights on the shorter wavelength's side due to the stimulated Raman scattering generating in signal lights on the longer wavelength's side is decreased (refer to paragraph 0010–0013 of the patent document 1).
A technique described in the patent document 2 relates to a pump light source unit, a Raman amplifier and an optical transmission system. An object thereof is to provide a structure which can adjust an output signal optical spectrum in an amplifying wavelength band. Any one of a plurality of pump lights is given a variable output pump light wavelength, whereby the pump light spectrum in the Raman amplification wavelength band can be adjusted (refer to paragraphs 0010–0011 of the patent document 2, for example). As shown in FIG. 13 in the patent document 2, two couples of pump lights are polarization-combined by two polarization combiners, respectively, and are depolarized by respective depolarizers to match the state of polarization of the two couples of the pump lights after the two couples of the pump lights are polarization-combined by the two different polarization combiners, whereby the polarization dependency of the Raman amplification gain is reduced (refer to paragraphs 0103–0107 of the patent document 2).
Patent Document 1: Japanese Patent Laid-Open (Kokai) Publication No. 2001-44934
Patent Document 2: Japanese Patent Laid-Open (Kokai) Publication No. 2003-86871
It is very important to realize a wide-band, high-efficient Raman amplifier. When a wide-band amplifier is configured using a plurality of pump lights at different wavelengths as above, the gain is controlled by varying the pump light power or varying the pump light wavelength as done in the technique in the patent document 2.
However, the gain characteristic generally has a certain width (band) with respect to a wavelength. For this, it is difficult to independently control the gain at a wavelength or in a wavelength band of a specific signal light (to control the gain without affecting the wavelength band of another signal light).
When a plurality of pump lights, in which neighboring pump lights are polarization-combined and depolarized in order to reduce the polarization dependency, are used for amplification in a system using a Raman amplifier as done in the patent document 2, the pump light power in the shorter wavelength's side is decreased due to an effect of pump lights at longer wavelength's side.
Further, the Raman effect occurs among the pump lights and the pump light power on the shorter wavelength's side is decreased by an effect of the pump lights in the longer wavelength's side, which causes difficulty with efficient amplification. This can be relieved by polarization-combining the signal light at a longer wavelength and the signal light at a shorter wavelength so that polarizations of the signal lights are orthogonal as described in the patent document 1. However, it is still difficult to independently control the gain at a wavelength or in a wavelength band of a specific signal light. Namely, even if the technique of the patent document 2 is simply applied to the technique of the patent document 1, it is extremely difficult to independently control the gain at a wavelength or in wavelength band of a specific signal light.